This application relates to solid state radiation imagers having imaging pixels including a photosensor and a thin film transistor (TFT) for selectively coupling the photosensor to a data line, and in particular to such imagers having a common dielectric layer for the transistor and photosensor components and the method of fabricating such an imager, which method reduces the number of steps necessary to fabricate an imager array.
Solid state imagers typically include a photosensor array coupled to a scintillating medium. Radiation absorbed in the scintillator generates optical photons which in turn pass into a photosensor, such as a photodiode, in which the optical photon is absorbed and an electrical signal corresponding to the incident optical photon flux is generated. Substantially hydrogenated amorphous silicon (a-Si) is commonly used in the fabrication of photosensors due to the advantageous photoelectric characteristics of a-Si and the relative ease of fabricating such devices. In particular, photosensitive elements, such as photodiodes, can be formed in connection with necessary control or switching elements, such as thin film transistors (TFT's), in a relatively large array.
Imager and display arrays are typically fabricated on large substrates on which many components, including TFTs, address lines, and devices such as photosensors, am formed through the deposition and patterning of layers of conductive, semiconductive, and insulative materials. The array is comprised of pixels, with the address lines and associated TFTs being coupled together to enable the photosensor in each pixel of the array to be respectively addressed, so that, for example, the charge developed by each photosensor during a cycle of exposure to the incident radiation can be selectively read. The TFT fabrication process involves several patterning steps to produce the desired arrangement of a channel region between a source and a drain electrode with the desired arrangement of semiconductive material disposed between the electrodes and over the gate electrode. The TFT is electrically coupled to a respective photosensor, such as a photodiode, which is disposed to absorb incident photons and accumulate the resulting charge produced in the photodiode.
The conventional fabrication process for such an imaging array typically includes steps to first fabricate the TFT and then the photosensor. The steps of fabricating the TFT include the formation of a gate electrode;the formation of a TFT body, that is, the deposition of a gate dielectric layer followed by layers of semiconductive material (typically a-Si) and doped semiconductive material overlying the gate electrode and underlying the source and drain electrodes; and the formation of the source and drain electrodes with a channel region therebetween and formation of associated contacts to address lines (the various address lines are typically formed at the same time as the gate electrodes and source/drain electrodes are patterned). The TFT is then coated with a protective passivation coating prior to the fabrication of the photodiode. The a-Si for the photodiode body is then deposited and patterned; the presence of the protective passivation layer over the TFT assembly is necessary to ensure that a-Si portions of the TFT are not damaged by the patterning process for the photosensor island (as the etchants used to form the a-Si body would similarly etch TFT components if allowed to come in contact with such a-Si portions).
In the conventional array formation process, after formation of the photosensor island another passivation layer is deposited over the array to provide electrical insulation between the photosensor island (except at a predetermined contact area) and the common electrode of the photosensor array, which is disposed over the passivation layer. Further, the passivation layer over the photosensor passivates the a-Si based photosensor body and protects the a-Si photosensor from environmental conditions (such as moisture) that can degrade its performance and also protects the array from exposure to materials, such as solvents, used in remaining steps of the fabrication process. Typically a scintillator is disposed over the common electrode of the photosensor array to complete the imager structure. In the conventional fabrication process described above, formation of the complex imager structure requires as many as twelve photomasks, which is a relatively large number of processing steps for a thin film semiconductor structure (simpler liquid crystal displays, by contrast, are typically fabricated in processes requiring only about 5 masks). Each patterning step involving photomasking and etching increases the likelihood of inadvertent damage to the array from the processing and handling required and increases processing time, and thus it is desirable that fabrication processes have as few masking steps as feasible to achieve desired structures.
An object of this invention is to provide a method of fabricating an imager array having fewer fabrication steps and resulting in the elimination of several dielectric layers in the assembled array.
It is a further object of this invention to provide an imager having, in each pixel of the array, a structure with a single common dielectric layer comprising TFT gate dielectric and the photosensor island passivation.